Engine combustion condition detecting apparatus equipped with malfunction diagnosing apparatus

ABSTRACT

A failure of a detecting circuit such as an ion current detecting circuit for detecting a combustion condition of an engine can be diagnosed at a stage for detecting a combustion condition by a combustion condition detecting apparatus equipped with a malfunction diagnosing apparatus. The engine combustion condition detecting apparatus equipped with the malfunction diagnosing apparatus is arranged by a sensing circuit for sensing an ion current flowing through a combustion chamber, a misfire judging means for judging misfire occurred in the engine in response to the detection signal derived from the sensing circuit, and a malfunction judging means for judging malfunction of the sensing circuit.

BACKGROUND OF THE INVENTION

The present invention generally relates to an engine combustioncondition detecting apparatus equipped with a malfunction (failure)diagnosing apparatus. More specifically, the present invention isdirected to such an engine combustion condition detecting apparatusequipped with a malfunction diagnosing apparatus, capable of preventingan erroneous diagnosis of engine combustion conditions caused bymalfunctions of this engine combustion condition detecting apparatus.

To detect a combustion condition of an engine, it is important to judgeas to whether this combustion condition is under normal condition, orunder misfire condition. Conventionally, as an apparatus for detecting acombustion condition, a combustion pressure sensor is used to detectpressure within a combustion chamber. However, since this combustionpressure sensor is expensive, this sensor could not be mounted on eachof cylinders due to high cost reason. Accordingly, this combustionpressure sensor is installed only on a specific cylinder to measurecombustion pressure thereof. Then, the combustion conditions of theengine are detected by this measured combustion pressure.

Also, to judge an occurrence of misfire in an engine, a torque variationof this engine is acquired as a change in the number of revolutions, anda fluctuation in the engine speed is detected. In this case, an error isproduced in the variation of the engine speed unless disturbance isremoved based on drive condition. Thus, there is a risk that thefluctuation is erroneously detected.

Under such a circumstance, the following detecting technique has beenproposed in which since the combustion conditions within the combustionchambers of the engine are directly detected, the combustion conditionsof the engine and the misfire condition are detected. In other words,this conventional detecting technique is performed by measuring theamount of ions contained in the combustion gas within the combustionchambers by way of the ion current detecting means. The combustionconditions of the engine can be directly detected by using thisconventional detecting technique without detecting the change of theengine speed, while considering the disturbance and the combustionpressure sensor.

Japanese patent application Un-examined Publication No. JP-A-4-308360discloses the engine combustion condition detecting apparatus withemployment of the above-explained ion current detecting means.

On the other hand, this proposed combustion condition detectingapparatus is arranged by the ion current detecting circuit for detectingthe ion currents produced by the combustion within the respectivecylinders of the engine, and also the judging means for judging as towhether the internal condition of the cylinder during the ignitionoperation is under normal combustion, or under misfire condition byusing the ion current signal outputted from this ion current detectingcircuit. This ion current detecting circuit means is inserted into theconventional ignition circuit. There is no problem when this ion currentdetecting circuit means is operated under normal condition, namelywithout under malfunction condition. However, when this circuit isbrought into the malfunction condition, even when the ignition operationis performed under normal state, such an erroneous judgement is made inthe case that the misfire happens to occur. Moreover, there is anotherproblem that which circuit portion of the ion current detecting circuitmeans is under malfunction condition.

The present invention has been made to solve the above-describedproblems, and therefore, has an object to provide a combustion conditiondetecting apparatus equipped with a malfunction diagnosing apparatuscapable of diagnosing malfunction of a detecting apparatus such as anion current detecting circuit for detecting a combustion condition of anengine in a stage for detecting combustion conditions.

SUMMARY OF THE INVENTION

To achieve the above-described object, an engine combustion conditiondetecting apparatus equipped. with a malfunction diagnosing apparatus,according to the present invention, is comprised of: a sensing circuitfor sensing an ion current flowing through a combustion room; misfirejudging means for judging misfire occurred in the engine in response tothe detection signal derived from the sensing circuit; and furthermalfunction judging means for judging malfunction of the sensingcircuit.

Also, as a preferable embodiment of the combustion condition detectingapparatus equipped with the malfunction diagnosing apparatus accordingto the present invention, when the misfire judging means judges that themisfire happens to occur, the malfunction judging means judges as towhether or not the misfire happens to occur by calculating a variationin revolution numbers for each of cylinders under a predetermined drivecondition, whereby the malfunction of the sensing circuit is diagnosed.Furthermore, the malfunction judging means judges malfunction of anelement employed in the sensing circuit based upon a mutual relationshipbetween an input signal supplied to the sensing circuit and an outputsignal derived from the sensing circuit.

Also, as a preferable embodiment of the combustion condition detectingapparatus equipped with the malfunction diagnosing apparatus accordingto the present invention, the combustion condition detecting circuit isfurther comprised of monitoring/processing means; when the misfirejudging means judges normal combustion, the monitoring/processing meansforcibly stops an ignition signal under a predetermined drive conditionto thereby induce misfire; the misfire judging means again judges as towhether or not the misfire happens to occur; and the malfunction judgingmeans diagnoses the malfunction of the sensing circuit based on thejudgement result of the misfire judging means. Furthermore, thecombustion condition detecting circuit is further comprised ofmonitoring/processing means and external self-diagnosing means; when themisfire judging means judges normal combustion, themonitoring/processing means and the external self-diagnosing meansforcibly stop an ignition signal under a predetermined drive conditionto thereby induce misfire; the misfire judging means again judges as towhether or not the misfire happens to occur; and the malfunction judgingmeans diagnoses the malfunction of the sensing circuit based on thejudgement result of the misfire judging means.

The engine combustion condition detecting apparatus equipped with themalfunction diagnosing apparatus with the above-described arrangement,according to the present invention, is arranged by the malfunctionjudging means for judging the malfunction of the sensing circuit fordetecting the ion currents within the combustion chambers. As aconsequence, in such a case that since the sensing circuit is broughtinto the malfunction condition, the misfire judging means makes theerroneous judgement, the malfunction of the sensing circuit can becorrectly detected.

Also, the malfunction judging means acquires both the input signal tothe sensing circuit and the output signal from the sensing circuit, anddiagnoses the malfunction of the sensing circuit based upon the mutualrelationship between the input/output signals. The following malfunctionof the circuit elements in the sensing circuit can be individuallydetected. That is, the charge capacitor, the zener diode, the diode, andthe detecting resistor are opened, or shortcircuited.

Furthermore, the engine combustion condition detecting apparatusaccording to the present invention is further comprised ofmonitoring/processing means, the monitoring/processing means forciblystops an ignition signal under a predetermined drive condition tothereby induce misfire. When the misfire judging means judges the normalcombustion, the misfire judging means again judges as to whether or notthe misfire happens to occur; and the malfunction judging meansdiagnoses the malfunction of the sensing circuit based on the judgementresult of the misfire judging means.

Furthermore, the engine combustion condition detecting apparatusaccording to the present invention is further comprised ofmonitoring/processing means and external self-diagnosing means, themonitoring/processing means and the external self-diagnosing meansforcibly stop an ignition signal under a predetermined drive conditionto thereby induce misfire. When the misfire judging means judges thenormal combustion, the misfire judging means again judges as to whetheror not the misfire happens to occur; and the malfunction judging meansdiagnoses the malfunction of the sensing circuit based on the judgementresult of the misfire judging means.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference ismade of a detailed description to be read in conjunction with theaccompanying drawings, in which:

FIG. 1 is an overall structural diagram for schematically showing anengine installed with an engine combustion condition detecting apparatusequipped with a malfunction diagnosing apparatus, according to anembodiment mode of the present invention;

FIG. 2 schematically represents a control block diagram of the enginecombustion condition detecting apparatus equipped with the malfunctiondiagnosing apparatus shown in FIG. 1;

FIG. 3 indicates an ion current detecting circuit (sensing circuit)employed in the engine combustion condition detecting apparatus equippedwith the malfunction diagnosing apparatus shown in FIG. 2;

FIG. 4 is an explanatory diagram for explaining operations of the ioncurrent detecting circuit shown in FIG. 3;

FIG. 5 is a table for explaining malfunction conditions occurred whenthe ion current detecting circuit of FIG. 3 judges misfires;

FIG. 6 schematically represents malfunction conditions during normaljudgement by the ion current detecting circuit of FIG. 3;

FIG. 7 is a flow chart for describing a malfunction diagnosing operationby the engine combustion condition detecting apparatus equipped with themalfunction diagnosing apparatus shown in FIG. 2;

FIG. 8 indicates a timing chart (1) for explaining malfunctiondiagnosing operation of FIG. 7;

FIG. 9 indicates a timing chart (2) for explaining malfunctiondiagnosing operation of FIG. 7; and

FIG. 10 is a table for illustratively showing failure detectingconditions of circuit elements employed in the ion current detectingcircuit of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to drawings a description will be made of an enginecombustion condition apparatus equipped with a malfunction (failure)diagnosing apparatus, according to an embodiment of the presentinvention.

FIG. 1 schematically represents an overall arrangement of a controlsystem for an engine installed with an engine combustion conditiondetecting apparatus equipped with a malfunction diagnosing apparatus,according to this embodiment of the present invention.

An actuator for controlling output power of an engine 100 and varioussorts of sensors for detecting drive conditions is mounted on thisengine 100.

As these various sorts of sensors, the following sensors are mounted onthis engine 100. That is, there are an intake air flow meter 101 formeasuring an intake air flow of the engine 100; a coolant temperaturesensor 102 for measuring a temperature of cooling water of the engine100; a crank angle sensor 103 for measuring a crank angle; a phasesensor 104 for discriminating a cylinder; a reference sensor 105 forindicating a specific crank angle; a throttle sensor 106 for measuringan angle of a throttle valve; an O₂ sensor, or air fuel ratio sensor 107for measuring O₂ concentration of exhaust gas, and so on.

Also, as the actuators, there are provided an injector 111 forcontrolling a fuel amount; an igniter 112 for controlling ignition; anidling speed control valve 113 for controlling an intake air amountduring idling operation; a purge valve 114 for controlling a purgeamount from a canister; and so on.

Furthermore, an engine control unit 120 is provided on theabove-described engine 100. The above-described various sensors andvarious sorts of actuators are connected to this engine control unit120. In response to the values sensed by the various sorts of sensors,the engine control unit 120 may control actuation amounts of the varioussorts of actuators. A sensing circuit (ion current detecting circuit)210 is connected to a secondary circuit side of an ignition coil 301 ofan ignition plug 108 employed in the engine 102. An output circuit sideof this sensing circuit 210 is connected to the engine control unit 120.

Then, the engine control unit 120 counts a quantity of pulses derivedfrom the crank angle sensor 103 within a predetermined time period so asto measure the number of revolutions of the engine 100, and alsocalculates a load given to the engine 100 based on an intake air amount.The engine control unit 120 also acquires rotation numbers of a wheel121a of an automobile, or a vehicle from a rotation number sensor 121 inorder to input therein a vehicle speed (VSP).

Also, the engine control unit 120 is internally equipped with acombustion condition detecting apparatus (will be discussed in detail)for the engine 100, and also equipped with a terminal 122 of an externalself-diagnosing apparatus capable of stopping the fuel injection andignition of the engine 100 with respect only to a specific cylinder.

A warning lamp 123 is arranged in such a manner that since this warninglamp 123 is turned ON in response to the detection result derived fromthe combustion condition detecting apparatus, this warning lamp 123 maywarn a driver of the vehicle. At the same time, this warning lamp 123may be arranged to indicates such a warning information an aself-diagnostic function tester.

FIG. 2 schematically indicates a control block diagram of the enginecontrol unit 120 containing the combustion condition detecting apparatusaccording to this embodiment mode. The engine control unit 120 includesa microcomputer.

The engine control unit 120 is internally provided with engine drivecondition inputting/processing unit 201; drive conditiondetecting/control amount calculating unit 202; output control unit 203;monitoring/diagnostic processing unit 204 for input/output of drivecondition; A/D converter 205 for an ion current; misfire judging unit206; sensing circuit malfunction judging unit 207; and a turn-oncondition unit 208.

The engine drive condition inputting/processing unit 201inputs/processes sensor signals via either the A/D converter or a periodmeasuring means, and these sensor signals are derived from the varioussorts of sensors mounted on the engine 100, such as the intake air flowmeter 101, the coolant temperature sensor 102, the crank angle sensor103, the phase sensor 104, the reference sensor 105, the throttle sensor106, and the 02 sensor.

Also, the detections of the combustion conditions within the combustionchambers of the engine 100 imply that ion currents are detected whichare produced during the combustion within the combustion chambers of theengine 100. The measurement of the ion currents is carried out in thesensing circuit unit (ion current detecting unit) 210. An output signalof this sensing circuit unit 210 is entered into the drive conditioninputting/processing unit 201, and on the other hand, is connected viathe A/D converter 205 to the misfire judging unit 206.

In the engine drive condition detecting/control amount calculating unit202, a load factor of the engine 100 and a necessity of fuel cutting arecalculated based on the input signals from the various sorts of sensors,and further control parameters such as a fuel injection amount and aignition timing are calculated. The output control unit 203 converts thefuel jetting amount into an actual fuel injection time width so as todrive the injector 111. Also, this output control unit 203 sets ignitiontiming based on the crank angle and time sensed from the referencesensor 104 to thereby output this set ignition timing to the ignitionplug 115.

Based upon the value of the ion current derived from the sensing circuitunit 210, the misfire judging unit 206 judges the drive condition of theengine, namely judges as to whether or not a misfire happens to occur ineach of the cylinders in the engine 100. Then, this misfire judging unit206 outputs the output signal of this judgement result to themonitoring/processing unit 204 and to the malfunction judging unit 207of the sensing circuit.

The monitoring/diagnostic processing unit 204, monitors a relationshipbetween the drive conditions and control amounts of the engine 100 inresponse to the output signals derived from the various sorts ofsensors, and also the output signal derived from the misfire judgingunit 206 so as to continuously perform the monitoring/diagnostic processcapable of detecting abnormal conditions of the various sensors and alsothe abnormal contents of the control operations. The monitored resultmay cause the output control unit 203 to give influences to the outputcontrols, for instance, the fuel cutting control. Also, thismonitoring/diagnostic processing unit 204 may be arranged in such amanner that an output signal is produced from the output control unit203 in response to a mutual communication signal between theabove-described output signals and the output signal from the externalself-diagnosing apparatus 211 externally provided with the enginecontrol unit 120.

In the sensing circuit malfunction judging unit 207, the malfunction ofthe sensing circuit 210 for the engine 100 is diagnosed. Thismalfunction diagnosing operation by the sensing circuit malfunctionjudging unit 207 may be carried out as follows. In response to either apreselected drive condition or the signal derived from the externalself-diagnosing apparatus 211, this sensing circuit malfunction judgingunit 207 judges as to whether or not an abnormal operation happens tooccur in the detection circuit of the sensing circuit unit 210, and alsoas to whether or not the internal component is opencircuited, orshortcircuited by considering the relationship between the judgementresult of the combustion condition in the misfire judging unit 206, andother drive conditions. When this sensing circuit malfunction judgingunit 207 detects that the abnormal operation happens to occur, thisdetection signal is compared with the output signal derived from theturn-ON condition 208 to thereby turn ON the warning lamp 123, so thatthe driver of the vehicle is required to repair this malfunction.

Also, in order to diagnose failures of the respective circuit elementsprovided in the sensing circuit 210, the ion current appearing on theinput side of the sensing circuit 210 is entered via the A/D converter205 to the malfunction judging unit 207 of the sensing circuit. Sinceboth this input signal and the output signal of the sensing circuit 210are additionally supplied to the malfunction judging unit 207, thefailures of the circuit elements themselves provided in the sensingcircuit 210 are detected.

FIG. 3 represents the sensing circuit (ion current detecting circuit)210 for measuring the ion current.

Within the combustion chambers of the engine 100, combustion gas isionizable to generate ions due to high-temperature heats produced duringcombustion operations. When a potential is produced within thiscombustion chamber, plus ions are attracted to a minus polarity sidewhereas minus ions are attracted to a plus polarity side. As a result,these ions may be detected as a current which is direct proportional toan amount of ions.

A circuit arrangement made of a zener diode 302, a charge capacitor 303,a diode 304, and a detecting resistor 305 is interposed between theground and a terminal on a cold side in the secondary winding side ofthe ignition coil 301. The terminal VB of the primary winding of theignition coil 301 is connected to the battery voltage terminal (a hotside).

Referring now to FIG. 3 and FIG. 4, operations of the sensing circuit(ion current detecting circuit) for measuring the above-described ioncurrent will be explained. When an ignition signal (see FIG. 4, (a)) isapplied to the ignitor 112 of the engine 100, a current (see FIG. 4,(b)) starts to flow through the ignition coil of the primary windingside of the ignition coil 301. When this ignition signal is returned tozero after a preselected energizing time period has passed, the currentflowing through the ignition coil of the primary winding side. Theenergy stored in the ignition coil 310 may produce a high voltage (seeFIG. 4, (c)) across the secondary winding coil, and then this highvoltage may produce spark (see FIG. 4, (d)) at the ignition plug 115, sothat the combustion of the engine is commenced.

The charge capacitor 303 is charged by the current flowing when thespark is generated, and this charging operation is continued until thecharged voltage becomes a zener voltage (see FIG. 4, (e)) of the zenerdiode 302. When the spark disappears, the charge current becomes zero.However, the voltage applied to the charge capacitor 303 is applied tothe ignition plug 107, so that a discharge current may flow through theignition coil 301 and the peripheral portion of the ignition plug 115.Thereafter, such an ion current (see FIG. 4, (f)) flows which isdirectly proportional to an amount of ions existing in the combustionchamber. Thus, the combustion conditions can be grasped by integratingthis ion current by the engine control unit 120.

When misfire happens to occur, since no combustion is made, ions are notgenerated, so that a waveform corresponding to an ion current is notproduced (namely, dotted line of FIG. 4, (f)) from the ion currentdetecting circuit 210. Conversely, when combustion is normally carriedout, an ion current waveform (namely, solid line of FIG. 4, (f)) isproduced. As a consequence, the misfire judging unit 206 can judge as towhether the normal combustion is made, or the misfire happens to occurby judging as to whether or not the ion current waveform is present. Inother words, while the discharge start timing is set as the starttrigger, the output signal from the ion current detecting circuit 210 isacquired by the misfire judging unit 206 employed in the engine controlunit 120 after a predetermined time period has passed. Then, this outputsignal is compared with a threshold value. If this output signal issmaller than, or equal to the threshold value, then, the misfire judgingunit 206 judges that the misfire happens to occur.

FIG. 5 is a table for representing malfunction (failure) conditions ofcircuit elements employed in the ion current detecting circuit unit(sensing circuit unit) 210. The judgements of these element malfunctionconditions are carried out in the malfunction judging unit 207 of thesensing circuit. Other than these element malfunction conditions, thismalfunction judging unit 207 judges that misfire happens to occurirrelevant to the combustion conditions in such a case that the inputline of the ion current detecting circuit unit 210 is disconnected,VB-shortcircuits, or ground-shortcircuited. The VB-shortcircuit meansthat a terminal of sensor or circuit is short-circuited to the VBterminal (battery voltage line). The ground-shortcircuit means that aterminal of sensor or circuit is short-circuited to the ground line.

Also, if the trigger signal, or the integration circuit for detectingthe ion current is not present, then the output signal becomes zero,which implies that the misfire similarly happens to occur. Even when theoutput of the ion current detecting unit 210 is ground-shortcircuited,it may be judged that the misfire happens to occur. The malfunctionjudging unit 207 may judge that the misfire always happens to occur evenwhen the charge capacitor 303 is opened, or shortcircuits, and furtherthe zener diode 302 is shortcircuited, and also either the diode 304 orthe detecting resistor 305 is shortcircuited. In the case that themalfunction judging unit 207 detects the occurrence of the misfireirrelevant to the combustion condition, the malfunction judging unit 207discriminates this misfire from the failure condition of the circuit,compared with the drive condition of the engine.

As shown in FIG. 6, if the malfunction judging unit 207 judges theoccurrence of the misfire though the time interval of the sensor signaloutputted from the reference sensor 105 is constant and further theengine revolutions are under normal condition, this malfunction judgingunit 207 judges that the ion current detecting circuit unit 210malfunctions. On the other hand, when the output of the ion currentdetecting circuit unit 210 is VB-shortcircuited, this output iscontinuously increased larger than, or equal to a threshold value. Thus,this condition is judged as the normal combustion. Also, in such a casethat the zener diode 302 is opened, the diode 304 is opened, and thedetecting resistor 305 is opened in the ion current detecting circuitunit 210, since the outputs from this ion current detecting circuit unit210 are increased, these conditions are judged as the normal combustioneven if the drive condition is brought into the misfire condition.

In the case that the normal combustion is detected under such a drivecondition that the misfire surely occurs, it is recognized that the ioncurrent detecting circuit unit 210 is brought into the malfunctioncondition. For instance, if the normal combustion is detected while thefuel supply is cut from the high engine speed, then this condition is sojudged as the malfunction of the ion current detecting circuit unit 210.

In the case that even if the ignition operation is stopped, thiscondition is judged as the normal combustion, it is predictable that atleast the harness is possibly VB-shortcircuited. As the possible driveconditions under which the ignition operation is stopped, there are theabove-described fuel supply cutting operation, and also tests executedin a repair factory where exhaust gas of an automobile can be properlyprocessed.

In this case, a fuel injection from a specific cylinder may beperformed, and ignition operation may be stopped, and further themalfunction judging unit may judge as to whether or not misfire happensto occur by combining input signals with each other, derived from theterminals of the external self-diagnosing apparatus 211, or combiningthe communication signals with each other, derived from theself-diagnosing tester.

FIG. 7 is a flow chart for describing malfunction diagnosing operations.First, at a step 601, a current misfire judgement is fetched so as tojudge as to whether or normal combustion judgement, or the misfirejudgement is made.

If the normal combustion is judged, then the process operation isadvanced to flow operations defined after a step 610. If the occurrenceof the misfire is judged, then the process operation is advanced to astep 602 at which the time interval of the sensor signals from thereference sensor 105 is measured. Then, the process operation isadvanced to a step 603.

At this step 603, the time interval measuring condition of the referencesensor 105 is checked. In other words, such a judgement is made as towhether or not the present drive condition becomes such a stablecondition that while the engine speed is within a preselected valuerange, there is substantially no variation in the vehicle speed (VSP)and the engine load.

At a further step 604, when a predetermined condition is satisfied, thereference sensor interval of the cylinder where the occurrence of themisfire is detected is compared with the reference sensor interval ofanother cylinder. If a difference between these intervals is smallerthan, or equal to a preselected value, then the process operation isadvanced to a step 605. At this step 605, the following judgement ismade. The engine itself is not brought into the misfire condition, buteither the ion current detecting circuit or the harness is brought intothe malfunction condition. Conversely, when the difference between theintervals is larger than, or equal to a predetermined value, the processoperation is advanced to a step 606 at which it is so judged hat themisfire happens to occur in the engine.

A process operation defined from a step 610 to a step 618 corresponds toa flow operation in the case that an engine condition is judged as thenormal combustion, namely is used to judge combustion conditions of theengine.

At a first step 610, a judgement is made as to whether or not the enginecombustion condition corresponds to the fuel supply cutting condition.If this engine combustion condition corresponds to the fuel supplycutting condition (fuel cut), then the process operation is advanced toa step 611. At this step 611, the duration time of this fuel supplycutting operation is measured. Thereafter, the waiting condition iscontinued until a preselected time period has passed. After thispreselected time period has passed, the process operation is advanced toa step 612. At this step 612, the output derived from the ion currentdetecting circuit unit 210 is acquired. If it is so judged that theengine combustion condition is still under normal combustion condition,then the process operation is advanced to a step 613. At this step 613,it is so judged that the ion current detecting circuit 210 is broughtinto the malfunction, or the harness is troubled.

At a step 614, a check is made as to whether or not there is such asignal produced from the external self-diagnosing apparatus of theengine control apparatus. If there is such a signal, then the processoperation is advanced to a step 615 at which the below-mentioneddiagnosing operations are carried out in a facility where the exhaustgas is properly processed.

That is, when an instruction to stop the ignition operation is issued bycombining the external signals with each other, or by combining theexternal signals with the signal derived from the self-diagnosingapparatus, the process operation is advanced to a step 615. At this step615, the ignition operation of the cylinder instructed at the previousstep 614 is stopped.

In such a case that even when the ignition operation of the designatedcylinder is stopped, the output signal from the ion current detectingcircuit is judged as the normal combustion (namely, no misfire), theprocess operation is advanced to a step 618 at which this condition isjudged as the harness failure. Conversely, when it is so judged that themisfire happens to occur at the step 616, the process operation isadvanced to a step 617. At this step 617, it is so judged that theelement employed in the ion current detecting circuit is opened. It isalternatively possible to cut the supply of fuel at the same time,depending upon a combination of signals, so that exhaust gas gives noadverse influence a peripheral environment.

FIG. 8 represents a timing chart for explaining the flow operationsdefined from the above-described steps 600 to 606. When the combustioncondition becomes stable, a signal interval Tref of a reference sensorsignal is measured. This signal interval Tref is equal to a timeinterval defined from one rising edge of a reference sensor signal tothe next rising edge of the reference sensor signal. This signalinterval is defined by capturing a free-running counter to calculate adifference between the presently captured value and the previouslycaptured value. If the free-running counter is initialized to zero bythe capture, then the captured value itself becomes a time difference.

The signal interval Tref is measured with respect to each of thecylinders, and then interval differences among the cylinders arecalculated. The interval differences may be simply calculated fromdifferences. Alternatively, if the interval differences are calculatedfrom the data of the plural cylinders, then the adverse influences ofthe revolution variations of other cylinders can be reduced.

When the interval difference is smaller than, or equal to apredetermined value, the engine 100 is driven under normal condition.Even if it is so judged that the misfire happens to occur, then eitherthe harness or the ion current detecting circuit means 210 is broughtinto the malfunction condition.

When the malfunction conditions are detected one time or continuouslydetected more than one time, the warning lamp 123 of the engine controlunit 120 is turned ON, which may instruct the automobile driver toreplace the ion current detection module at an earlier stage.

FIG. 9 shows a timing chart for explaining the flow operation definedfrom the above-described steps 610 to 618.

A fuel-supply cutting flag under engine control is checked. If the fuelsupply is cut, then a waiting time timer is initiated. If the fuelsupply is not cut, then the waiting time timer is set to an initialvalue thereof. When the waiting time timer reaches a preselected value,the output of the ion current detecting circuit unit 210 is monitored.Even in this condition, when it is so judged as the normal combustion,then either the harness or the ion current detecting circuit unit 210 isbrought into the malfunction condition.

When the malfunction conditions are detected one time or continuouslydetected more than one time, the warning lamp 123 of the engine controlunit 120 is turned on, which may instruct the automobile driver toreplace the ion current detection module at an earlier stage.

FIG. 10 is a table for explaining judgements of element failures withinthe ion current detecting circuit unit 210.

The ion current detecting circuit unit 210 employs four elements. Thereis a problem in the above-described diagnosing unit such that theelement under failure state cannot be specified. To avoid this problem,a monitor circuit for monitoring an input and an output of a circuit isadditionally provided to detect an element failure.

Reference numeral 901 of FIG. 10 indicates such a malfunction case thatan input signal is different from an output signal, and a voltage equalto the zener voltage of the zener diode 304 appears in the input signal.This failure implies that the charge capacitor 303 is opened. If theoutput signal becomes constant, i.e., zero, then either the diode or thedetecting resistor is shortcircuited. In this malfunction case, nodiscrimination is made, namely either the diode is shortcircuited, orthe detecting resistor is shortcircuited.

Reference numerals 902 and 904 indicate such a malfunction case that ifthe input signal is identical to the output signal, and the integratedoutput from the ion current detecting circuit unit 210 becomes zero, thezener diode 302 is shortcircuited, or the charge capacitor 303 isshortcircuited. Also, in this malfunction case, no discrimination ismade, namely either the zener diode 302 is shortcircuited, or the chargecapacitor 303 is shortcircuited.

Reference numeral 903 indicates such a failure case that the signal forcharging the output signal along the minus direction is integrated, andthus the zener diode 303 is opened. It should be noted that referencenumeral 903 indicates such a malfunction case that the high voltagegenerated at the secondary winding side of the ignition coil isrepeatedly applied to the charge capacitor 303 even in the instantaneousmanner. As a result, there is a problem in the withstanding voltage ofthe charge capacitor 303. Also, since a high voltage spike is appliedduring the measurement of the input signal, it is preferable that A/Dinputs are protected.

In the case that two sets of ignition coils are measured by one set ofsuch an ion current detecting circuit unit 210, since ignition operationis carried out via another ignition coil, the same operation as that ofthe simultaneous ignition mode is performed. As a result, although theignition voltages are lowered, there is no adverse influence in theignition performance.

Reference numeral 906 indicates such a malfunction case that when theoutput signal becomes plus, the zener diode 302 is opened. Referencenumerals 906 and 908 are such malfunction cases that when the outputsignals are zero, either the zener diode 302 is shortcircuited, or thedetecting resistor 305 is shortcircuited.

Reference numeral 907 indicates such a malfunction case that when theoutput signal becomes minus, especially when the ignition signal iscommenced, the detecting resistor 305 is opened.

While one embodiment mode of the present invention has been described indetail, the present invention is not limited to the above-describedembodiment mode, but may be modified, or changed without departing fromthe technical spirit/scope of the present invention defined in thepending claims thereof.

As apparent from the above-described explanations, since the enginecombustion condition detecting apparatus equipped with the malfunctiondiagnosing apparatus, according to the present invention, is comprisedof the sensing circuit malfunction judging unit for detecting the ioncurrents flowing through the combustion chambers, the malfunction ofthis sensing circuit can be diagnosed on the board.

Also, the malfunction judging unit can detect the failures of therespective elements employed in the sensing circuit based upon the inputsignal supplied to the sensing circuit, and also the output signalderived from this sensing circuit.

Furthermore, the ignition signal is forcibly stopped under a preselecteddrive condition by the monitoring/processing unit so as to induce themisfire. As a consequence, when the misfire judging unit judges thenormal combustion, the misfire is induced. This misfire judging unitagain judges as to whether or not the misfire happens to occur. As aresult, the misfire judging unit can diagnose as to whether or not thesensing circuit is brought into the malfunction condition.

What is claimed is:
 1. An engine combustion detecting apparatuscomprising:a sensing circuit for sensing an ion current flowing througha combustion chamber; misfire judging means for judging occurrence ofmisfire in the engine in response to a detection signal derived fromsaid sensing circuit; and malfunction judging means for judgingmalfunction of said sensing circuit.
 2. An engine combustion detectingapparatus as claimed in claim 1, whereinwhen said misfire judging meansjudges that the misfire happens to occur, said malfunction judging meansjudges as to whether or not the misfire happens to occur by calculatinga variation in number of revolution for each of cylinders under apredetermined drive condition, whereby the malfunction of said sensingcircuit is diagnosed.
 3. An engine combustion detecting apparatus asclaimed in claim 1, whereinsaid malfunction judging means judgesmalfunction of an element employed in the sensing circuit based upon amutual relationship between an input signal supplied to said sensingcircuit and an output signal derived from said sensing circuit.
 4. Anengine combustion detecting apparatus as claimed in claim 1, whereinsaidcombustion condition detecting circuit is further comprised ofmonitoring/processing means; when said misfire judging means judgesnormal combustion, said monitoring/processing means forcibly stops anignition signal under a predetermined drive condition to thereby inducemisfire; said misfire judging means again judges as to whether or notthe misfire happens to occur; and said malfunction judging meansdiagnoses the malfunction of said sensing circuit based on the judgementresult of said misfire judging means.
 5. An engine combustion detectingapparatus as claimed in claim 1, whereinsaid combustion conditiondetecting circuit is further comprised of monitoring/processing meansand external self-diagnosing means; when said misfire judging meansjudges normal combustion, said monitoring/processing means and saidexternal self-diagnosing means forcibly stop an ignition signal under apredetermined drive condition to thereby induce misfire; said misfirejudging means again judges as to whether or not the misfire happens tooccur; and said malfunction judging means diagnoses the malfunction ofsaid sensing circuit based on the judgement result of said misfirejudging means.